(1) Field of the Invention
The present invention relates to an electrode plate for display devices such as color liquid crystal display devices, input and output devices using liquid crystals, electroluminescence displays containing liquid crystals between transparent substrates and display drives using an electroluminescence material which become showable when voltage is applied thereto. Furthermore, the present invention also relates to a method for the preparation of the electrode plate for display devices.
(2) Description of the Prior Art
Recently, in high-density display devices such as liquid crystal display devices, there is a tendency such that the pitch of pixels or terminals decrease to a level as small as about 100 .mu.m and simultaneously screens are scaled up. Therefore, it is necessary to lower the resistance of transparent electrodes and to decrease the thickness of the display devices, which requires the surface disposition of driving IC's. For the purpose of lowering the resistance of the electrodes, some techniques have been used. They are the technique of disposing, as an auxiliary conductor, a metallic conductor pattern having a width of several tens of micrometers on a part of each transparent electrode, and the technique of disposing a metallic conductor on the terminal of each substrate.
In an electrode plate for a display device, a substrate is coated with a color filter by means of a dyeing method, a printing method or an electrodeposition method, and the color filter is then covered all over with a transparent conductive layer, followed by etching to pattern the conductive layer, thereby forming transparent conductive electrodes. These techniques are known. In addition, in order to provide the color filter with acid resistance and to obtain the flat surface (e.g., unevenness on the surface =.+-.0.1 .mu.m or less) of the color filter, a color filter coating layer can be formed on the transparent electrodes. Furthermore, transparent electrodes can be formed on the color filter coating layer, and a surface coating layer can be formed on the transparent electrodes. These techniques are also known. Moreover, in order to electrically connect with the transparent electrodes in the terminal portion of the electrode plate for the display device with high reliability, a known technique is used in which the coating layer on the terminal portion is patterned with an organic solvent so that the coating layer may be left only on the color filter. As described in Japanese Patent Laid-open Publication No. 233720/1986, the color filter coating layer of a peptide resin such as glue, casein or gelatin may be patterned on the color filter. Moreover, the color filter coating layer may be formed from acrylurethane acrylate resin which is different from the peptide resin, silicon, polyimide or the like, and this technique is also known.
As other examples of using the coating layer, there are the method in which the color filter coating layer on the color filter is patterned with an organic solvent in order to permit the surface disposition of IC's for the display device, and the flip-chip method in which plural IC pads are plated with a soft solder so that the thickness of the plate may be about several tens of micrometers, and they are soldered face downward directly on metallic conductors on the electrode plate for the display device. In this case, members which project from the surface of the substrate, i.e., the so-call solder dams are formed from polyimide around the metallic conductor patterns to prevent the solder from overflowing out of defined areas, whereby the metallic conductors are prevented from electrically shorting.
As materials for the color filter coating layer, a variety of resins can be used.
Japanese Patent Laid-open Publication No. 233720/1986 discloses the dyed coating layer of a polypeptide resin such as glue, casein or gelatin. However, this coating layer is poor in chemical resistance. For example, this dyed coating layer of gelatin commences to be attacked with a polyimide solution containing N-methylpyrrolidone in about one minute, a 2% aqueous alkaline solution in about 30 seconds, and a 20% hydrochloric acid in about one minute. Therefore, in the process in which a transparent conductive layer or an oriented film (polyimide or the like) on the color filter coating layer is patterned, the color filter coating layer is impaired with the developing solution containing an organic solvent such as N-methylpyrrolidone, so that productivity deteriorates. In addition, the disclosed color filter coating layer is also poor in humidity resistance, and for example, the coating layer commences to be decolored at 40.degree. C. at a humidity of 90% in 100 hours, which lowers productivity. In particular, the color filter prepared by the dyeing method is liable to be easily decolored and discolored by a heat treatment after the formation of the transparent conductive layer, and the limit of its heat resistance is 180.degree. C. which is unplactical. Furthermore, the flatness on the surface of the color filter is poor, about .+-.0.1 .mu.m.
Acrylic resin and urethane acrylate resin have higher chemical resistance than gelatin. However, the limit of the heat resistance of these resins is 180.degree. C, and therefore wrinkles, blisters and cracks easily occur under the influence of the subsequent heat treatment. In particular, the color filter prepared by the dyeing method is easily discolored and decolored by the heat treatment after the formation of the transparent conductive layer. Moreover, when the acrylic resin or the urethane acrylate resin is used, the flatness on the surface of the color filter is insufficient, about .+-.0.3 .mu.m.
Silicone resin and polyimide resin are excellent in chemical resistance and heat resistance. However, when the usual silicone resin or polyimide resin is used, it is difficult to perform a patterning operation in accordance with a photolithography method. In addition, when the substrate is coated with such a heat-resistant resin all over, the resin layer is to be interposed between the transparent electrodes or the metallic conductors and an inorganic oxide layer or the substrate. Therefore, when pressure is applied from above, the transparent electrodes are easily damaged, electrical joint portions between the end of the electrode plate and pixels are cut, and even if not cut, resistance in the electrical joint portions heightens, so that the performance of the display deteriorates. Furthermore, when the silicone resin or the polyimide resin is used, the flatness on the surface of the color filter is as poor as about .+-.0.3 .mu.m. The photosensitive polyimide which can be patterned by the photolithography method is extremely expensive, and its price in Japan is 180,000 yen per kilogram.
The surface coating layer and the solder dams have been formed from polyimide resin. However, the latter is extremely expensive as mentioned above, and in the case that the polyimide is patterned, it is necessary to employ solvent development or dry etching. In addition, when the solvent development is carried out by the use of the photosensitive polyimide, undeveloped portions are apt to be left on the surface of the substrate, and lead wires are easily cut when disposition is made in the terminal portion of the substrate, with the result that reliability is lost.
Since the above-mentioned electrode plate for the display device is prepared integrally, one defect has an influence on the whole of the electrode plate. Therefore, the greater the area of the electrode plate, the higher the probability of fault formation. This probability is proportional to the number of defective articles, and thus it is difficult to prepare the electrode plates for the display devices having large screens where the probability of the fault formation is high.
In view of the above-mentioned situations, materials for the color filter coating layer and the surface coating layer are demanded which are inexpensive, can be safely used in a development process, can protect layers from solvent used in the development process, and have a heat-resistant temperature of 220.degree. C. or more, humidity resistance and chemical resistance. Furthermore, on the surface of the color filter, a flatness of .+-.0.15 to .+-.0.2 .mu.m which is in the practical range is demanded, and particularly in the case that operation is performed under conditions of a high duty, maintaining high contrast and uniform display performance, as in STN liquid crystal devices and homeotropic liquid crystal devices, a flatness of .+-.0.1 .mu.m or less is desired.